Congo Red Staining for Amyloid Plaques

Congo red stain applied to the tissue gives the amyloid protein a salmon-pink color and when placed under polarized light, the amyloid proteins have an apple-green birefringence. This apple-green birefringence is considered pathognomonic for amyloid fibril deposits (Howie et al., 2008). Congo red was purchased from Sigma–Aldrich (cat number C6277). It is an organic compound, a sodium salt of 3,3′-[(1,1′-biphenyl)-4,4′-diyl]bis(4-aminonaphthalene-1-sulfonic acid) and is an azo dye. Congo red is water-soluble, yielding a red colloidal solution. Paraffin fixed human brain sections (n = 6, in glass slides) were deparaffinized and hydrated with iron free distilled water. The brain sections were then immersed in Congo red dye for 10 min and rinsed thereafter in distilled water. The sections were differentiated quickly (5–10 dips) in alkaline alcohol solution and rinsed in tap water. Slides were counterstained in Gill’s hematoxylin for 30 s and mounted with a synthetic resin medium.